Stereoselective synthesis of vitamin D analogues

ABSTRACT

The present invention relates to intermediates useful for the synthesis of calcipotriol or calcipotriol monohydrate, to methods of producing said intermediates, and to methods of stereoselectively reducing said intermediates.

This Non-provisional application claims priority under 35 U.S.C. §119(e)on U.S. Provisional Application No(s). 60/553,962 filed on Mar. 18, 2004and under 35 U.S.C. §119(a) on Patent Application No(s). PA 2004 00454filed in Denmark on Mar. 22, 2004, the entire contents of which arehereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to methods of producing calcipotriol{(5Z,7E,22E,24S)-24-cyclopropyl-9,10-secochola-5,7,10(19),22-tetraene-1α-3β-24-triol}or calcipotriol monohydrate by stereoselective reduction. The presentinvention further provides novel intermediates and methods for thesynthesis of the intermediates useful for producing calcipotriol orcalcipotriol monohydrate.

BACKGROUND OF THE INVENTION

Calcipotriol or calcipotriene (structure I) [CAS 112965-21-6] shows astrong activity in inhibiting undesirable proliferation of epidermalkeratinocytes [F. A. C. M. Castelijins, Gerritsen, I. M. J. J. vanVlijmen-Willems, P. J. van Erp, P. C. M. van de Kerkhof; Acta Derm.Venereol. 79, 11, 1999]. The efficiency of calcipotriol and calcipotriolmonohydrate (II) in the treatment of psoriasis was shown in a number ofclinical trials [D. M. Ashcroft et al.; Brit. Med. J. 320, 963-67, 2000]and calcipotriol is currently used in several commercial drugformulations.

In the preparation of calcipotriol, the specific stereochemistry for thehydroxyl group at C-24 is necessary for full expression of thebiological activity. Under current methodology, the requiredstereochemistry is introduced by one of the following methods:

-   (i) non-diastereoselective reduction of C-24 keto-trienes followed    by the separation of diastereomeric mixtures of the C-24 hydroxyl    epimers obtained via chromatography (WO 87/00834 & M. J. Calverley;    Tetrahedron, 43 (20), 4609-19, 1987);-   (ii) attachment of an enantiopure C-24-hydroxyl-carrying side chain    to the vitamin D skeleton (M. J. Calverley, Synlett, 157-59, 1990);-   (iii) selective enzymatic esterification of one of C-24 hydroxyl    epimers followed by chromatographic separation (WO 03/060094).

The non-diastereoselective reduction of C-24 keto-trienes followed bychromatographic separation of the epimeric mixture (i) is the mostwidely practiced procedure for obtaining the desired epimer. Thisreduction process yields mainly the undesired C-24 epimeric alcohol(typically about 60% of the unwanted 24-R epimer) and it is difficult toseparate the desired S-epimer from such a mixture by chromatography on aproduction scale.

The stereoselective synthesis (ii) is still an unfavourable process forscale up due to its multi step nature and cost and due to the fact thattoxic intermediates are used. The enzymatic esterification process (iii)has the disadvantage, apart from the high cost of the enzymes employed,that it introduces, depending on the selectivity of the enzyme, 1-2additional reaction steps which adds even further costs to the process.

The stereoselective reduction of C-24 ketones directly to the desiredC-24 hydroxyl epimers has for example been described for cholesterolderivatives in WO 98/24800 and by M. Ishiguro et al., J. C. S. Chem.Comm., 115-117, 1981. The stereoselective reduction of a side chaintriple bond analogue of calcipotriol with unprotected triene systemusing S-alpine borane has been described by M. J. Calverly et al. inBioorg. Med. Chem. Lett., 1841-1844, 3(9), 1993.

A major technical problem of using stereoselective reduction methods forthe synthesis of calcipotriol stems from the fact that the unsaturatedtriene system of hitherto known intermediates for the synthesis ofcalcipotriol are chemically labile, such as towards Lewis acidicconditions, that they are relatively easily oxidised, and that they areusually not compatible with the typical reduction reaction conditionsemployed. This results in reduced yields, impure products and tediouswork-up procedures, especially on large-scale.

It is an object of this invention to provide an alternative process forthe synthesis of calcipotriol, which may overcome one or more of thevarious problems and disadvantages described above.

The present invention provides a novel process to producediastereomerically enriched C-24 hydroxyl epimers of calcipotriolderivatives using a novel synthetic pathway comprising a stereoselectivereduction step. The present invention further provides novel chemicallymore stable intermediates where the labile triene system is protected assulphur dioxide adduct. By producing diastereomerically enriched C-24hydroxyl epimers of calcipotriol derivatives the yield and the efficacyof the subsequent separation of the desired C-24 S-hydroxyl epimer maybe greatly improved.

SUMMARY OF THE INVENTION

It has surprisingly been found that compounds of general structure III,

wherein X represents either hydrogen or OR₂,and wherein R₁ and R₂ may be the same or different and representhydrogen, or a hydroxy protecting group,in an inert solvent with a reducing agent or with a reducing agent inthe presence of a chiral auxiliary,to give a mixture of compounds of general structure IVa and IVb,

which is enriched with IVa, wherein X, R₁, and R₂ are as defined above.

In a first aspect, this invention relates to a method for producingcalcipotriol{(5Z,7E,22E,24S)-24-cyclopropyl-9,10-secochola-5,7,10(19),22-tetraene-1α-3β-24-triol}or calcipotriol monohydrate comprising the steps of:

(a) reducing a compound of general structure III,

wherein X represents OR₂,

and wherein R₁ and R₂ may be the same or different and representhydrogen or a hydroxy protecting group,

in an inert solvent with a reducing agent or with a reducing agent inthe presence of a chiral auxiliary,

to give a mixture of compounds of general structure IVa and IVb, whichis enriched with IVa,

wherein X, R₁ and R₂ are as defined above;

(b) reacting the mixture of compounds of general structure IVa and IVb,which is enriched with IVa, in the presence of a base to give a mixtureof compounds of general structure Va and Vb, which is enriched with Va,

wherein X, R₁ and R₂ are as defined above;(c) separating the compound of general structure Va from the mixture ofcompounds of general structure Va and Vb which is enriched with Va,wherein X, R₁ and R₂ are as defined above;(d) isomerising the compound of general structure Va to the compound ofgeneral structure VIa,

wherein X, R₁ and R₂ are as defined above; and(e) when R₁ and/or R₂ are not hydrogen, removing the hydroxy protectinggroup(s) R₁ and/or R₂ of the compound of general structure VIa togenerate calcipotriol or calcipotriol monohydrate.

In a further aspect, this invention relates to a method for producingcalcipotriol or calcipotriol monohydrate comprising steps (a)-(b) aboveand further comprising the steps of:

(f) isomerising the mixture of compounds of general structure Va and Vb,wherein X, R₁ and R₂ are as defined in claim 2, which is enriched withVa, to a mixture of compounds of general structure VIa and VIb, which isenriched with VIa,

wherein X, R₁ and R₂ are as defined above;(g) separating the compound of general structure VIa from the mixture ofcompounds of general structure VIa and VIb which is enriched with VIa,wherein X, R₁ and R₂ are as defined above;(h) when R₁ and/or R₂ are not hydrogen, removing the hydroxy protectinggroup(s) R₁ and/or R₂ of the compound of general structure VIa togenerate calcipotriol or calcipotriol monohydrate.

In a still further aspect, this invention relates to a method forproducing calcipotriol{(5Z,7E,22E,24S)-24-cyclopropyl-9,10-secochola-5,7,10(19),22-tetraene-1α-3β-24-triol}or calcipotriol monohydrate comprising the steps of:

(j) reducing a compound of general structure III,

wherein X represents hydrogen,

and wherein R₁ represents hydrogen or a hydroxy protecting group,

in an inert solvent with a reducing agent or with a reducing agent inthe presence of a chiral auxiliary,

to give a mixture of compounds of general structure IVa and IVb, whichis enriched with IVa,

wherein X and R₁ are as defined above;

(k) reacting the mixture of compounds of general structure IVa and IVb,which is enriched with IVa, in the presence of a base to give a mixtureof compounds of general structure Va and Vb, which is enriched with Va,

wherein X and R₁ are as defined above;

(l) separating the compound of general structure Va from the mixture ofcompounds of general structure Va and Vb which is enriched with Va,wherein X and R₁ are as defined above;

(m) hydroxylating the compound of general structure Va with a suitablehydroxylating agent, wherein X and R₁ are as defined above to give acompound of general structure Va, wherein X represents OR₂ and R₂represents hydrogen, and wherein R₁ is as defined above;(o) isomerising the compound of general structure Va to the compound ofgeneral structure VIa,wherein X, R₁ and R₂ are as defined above; and(p) when R₁ is not hydrogen, removing the hydroxy protecting group R₁ ofthe compound of general structure VIa to generate calcipotriol orcalcipotriol monohydrate.

In a still further aspect, this invention relates to a method forproducing calcipotriol or calcipotriol monohydrate comprising steps(j)-(l) of claim 4 and further comprising the steps of:

(q) protecting the C-24 hydroxy group of the compound of generalstructure Va, wherein X represents hydrogen, and wherein R₁ representshydrogen or a hydroxy protecting group, with a hydroxy protecting group;

(r) hydroxylating the C-24 hydroxy protected compound of generalstructure Va with a suitable hydroxylating agent, wherein X and R₁ areas defined above to give a C-24 hydroxy protected compound of generalstructure Va, wherein X represents OR₂ and R₂ represents hydrogen, andwherein R₁ is as defined above;(s) removing the C-24 hydroxy protecting group of the compound ofgeneral structure Va;(t) isomerising the compound of general structure Va to the compound ofgeneral structure VIa,wherein X, R₁ and R₂ are as defined above; and(u) when R₁ is not hydrogen, removing the hydroxy protecting group R₁ ofthe compound of general structure VIa to generate calcipotriol orcalcipotriol monohydrate.

In a still further aspect, this invention relates to a method forproducing a compound of general structure III,

wherein X represents either hydrogen or OR₂,

and wherein R₁ and R₂ may be the same or different and representhydrogen, or a hydroxy protecting group,

by reacting a compound of general structure VII or VIII,

wherein R₁ and R₂ are as defined above,with sulphur dioxide.

In a still further aspect, this invention relates to a method ofreacting the mixture of compounds of general structure IVa and IVb,

wherein X represents either hydrogen or OR₂,

and wherein R₁ and R₂ may be the same or different and representhydrogen, or a hydroxy protecting group,

which is enriched with IVa, in the presence of a base to give a mixtureof compounds of general structure Va and Vb, which is enriched with Va,

wherein X, R₁, and R₂ are as defined above.

In a still further aspect, this invention relates to a method forproducing calcipotriol{(5Z,7E,22E,24S)-24-cyclopropyl-9,10-secochola-5,7,10(19),22-tetraene-1α-3β-24-triol}or calcipotriol monohydrate comprising any one of the methods above.

In a still further aspect, this invention relates to a compound ofgeneral structure IIIa or IIIb, or mixtures thereof,

wherein X represents either hydrogen or OR₂,and wherein R₁ and R₂ may be the same or different and representhydrogen, or a hydroxy protecting group.

In a still further aspect, this invention relates to a compound ofgeneral structure IVaa, IVab, IVba, IVbb, IVb, or mixtures thereof,

wherein X represents either hydrogen or OR₂,and wherein R₁ and R₂ may be the same or different and representhydrogen, or a hydroxy protecting group.

In a still further aspect, this invention relates to the use of acompound of general structure IIIa, IIb, IVaa, IVba, IVab, IVbb as anintermediate in the manufacture of calcipotriol or calcipotriolmonohydrate.

DETAILED DESCRIPTION OF THE INVENTION Definitions

As used herein a “hydroxy protecting group” is any group which forms aderivative that is stable to the projected reactions wherein saidhydroxy protecting group can be selectively removed by reagents that donot attack the regenerated hydroxy group. Said derivative can beobtained by selective reaction of a hydroxy protecting agent with ahydroxy group. Silyl derivatives, such as tert-butyldimethylsilylforming silyl ethers are examples of hydroxy protecting groups. Silylchlorides such as tert-butyldimethylsilyl chloride (TBSCl),trimethylsilylchloride, triethylsilylchloride,diphenylmethylsilylchloride, triisopropylsilylchloride, andtert-butyldiphenylsilylchloride are examples of hydroxy protectingagents. Hydrogen fluoride, such as aqueous HF in acetonitrile, or tetran-butylammonium fluoride are examples of reagents which can remove silylgroups. Other hydroxy protecting groups include ethers, such astetrahydropyranyl (THP) ether, including alkoxyalkyl ethers (acetals),such as methoxymethyl (MOM) ether, or esters, such as chloroacetateester, trimethylacetate, acetate or benzoate ester. Non-limitingexamples of hydroxy protecting groups and methods of protection andremoval, all included in the scope of this application, can for examplebe found in “Protective Groups in Organic Synthesis”, 3^(rd) ed., T. W.Greene & P. G. M. Wuts eds., John Wiley 1999 and in “Protecting Groups”,1^(st) ed., P. J. Kocienski, G. Thieme 2000.

As used herein, “alkyl” is intented to mean a linear or branched alkylgroup, which may be cyclic or acyclic, having one to twenty carbonatoms, preferably one to seven carbon atoms. The methyl group, ethylgroup, n-propyl group, isopropyl group, pentyl group, hexyl group, andthe tert-butyldimethyl group are non-limiting examples of alkyl groups.

As used herein “reducing agent” is intended to mean any agent capable ofreducing, including enantioselectively or diastereoselectively reducing,the C-24 keto group of a compound of general structure III to give acompound of general structure IV. In one embodiment, the reducing agentmay reduce the C-24 keto group of a compound of general structure IIIwithout a chiral auxiliary to yield a mixture of compounds of generalstructure IV, wherein said mixture is enriched for the desired epimerIVa (preferably yielding the 24-S isomer). In another embodiment, thereducing agent may reduce the C-24 keto group of a compound of generalstructure III in the presence of a chiral auxiliary to yield a mixtureof compounds of general structure IV, wherein said mixture is enrichedfor the desired epimer IVa (preferably yielding the 24-S isomer) Thereducing agent may be chiral or non-chiral. Examples of reducing agentsinclude, but are not limited to borane reducing agents, metallichydrides, such as lithium aluminium hydride, sodium borohydride, orAlH₃, optionally in the presence of lanthanide salts (e.g. LaCl₃, CeBr₃,CeCl₃), or NaBH₃(OAc), Zn(BH₄)₂, and Et₃SiH. Other reducing agentsinclude, but are not limited to, hydrogen in the presence of a catalyst,such as platinum or ruthenium, sodium in ethanol, isopropyl alcohol andaluminium isopropoxide, and zinc powder in water or alcohol.

As used herein “borane reducing agent” includes borane or any boranederivative, such as borane complexes with amines or ethers. Non-limitingexamples of borane reducing agents e.g. includeN,N-diethylaniline-borane, borane-tetrahydrofuran, 9-borabicyclononane(9-BBN), or borane dimethylsulfide.

As used herein, “chiral auxiliary” means any chiral compound oroptically active catalyst, e.g. a compound comprising asymmetricallysubstituted carbon atoms or axially chiral compounds, or mixtures ofchiral compounds and/or optically active catalysts, which will improvethe yield of a compound of general structure IVa with respect to itsepimer (increase the molar ratio IVa:IVb) in the reduction of a compoundof general formula III with said reducing agent. Said chiral auxiliarieswill thus be any compound which is capable of increasing thestereoselectivity, in the reduction reaction of a compound of generalstructure III in comparison to the yield or stereoselectivity for IVawithout the chiral auxiliary present or involved. Non-limiting examplesof chiral auxiliaries include chiral 1,2-amino-alcohols, such as chiralcis-1-amino-2-indanol derivatives, such as(1S,2R)-(−)-cis-1-amino-2-indanol, orcis-1-amino-1,2,3,4-tetrahydronaphthalen-2-ol, such as(1S,2R)-cis-1-amino-1,2,3,4-tetrahydronaphthalen-2-ol. Other examplesare binaphthyl derivatives, such as(R)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl-ruthenium acetate2,2′-dihydroxy-1,1′binaphthyl derivatives. Further examples include butare not limited to (R)-(+)-α,α-diphenyl-2-pyrrolidinmethanol,(R)-(+)-2-amino-4-methyl-1,1-diphenyl-1-pentanol,(R)-(−)-2-amino-3-methyl-1,1-diphenyl-1-butanol,(R)-(+)-2-amino-1,1,3-triphenyl-1-propanol, and(1R,2S)-(−)-2-amino-1,2-diphenyl ethanol.

As used herein, “inert solvent” means any organic solvent compatiblewith said reducing agent under the reaction conditions employed, ormixtures of such solvents. The choice of such solvent will depend on thespecific reducing agent used. Non-limiting examples of inert solventsinclude hydrocarbons, such as toluene, and ethers, such as tert-butylmethyl ether or tetrahydrofuran.

A mixture of compounds of general structure IVa and IVb, which isenriched with IVa, means a mixture, optionally comprising othercompounds or solvents, were the molar ratio (diastereomer ratio) ofIVa/IVb is one (50:50) or larger than one, thus that the mixturecontains at least 50% of the compound of general structure IVa(containing 50% or less of the compound of general structure IVb).

A mixture of compounds of general structure Va and Vb, which is enrichedwith Va, means a mixture, optionally comprising other compounds orsolvents, were the molar ratio (diastereomer ratio) of Va/Vb is one(50:50) or larger than one, thus that the mixture contains at least 50%of the compound of general structure Va (containing 50% or less of thecompound of general structure Vb).

A mixture of compounds of general structure VIa and VIb, which isenriched with VIa, means a mixture, optionally comprising othercompounds or solvents, were the molar ratio (diastereomer ratio) ofVIa/VIb is one (50:50) or larger than one, thus that the mixturecontains at least 50% of the compound of general structure VIa(containing 50% or less of the compound of general structure VIb).

As used herein, “separating a compound” includes the purification and/orisolation of a compound, e.g. to at least 90% purity, such as to atleast 95% purity, such as 97% purity, 98% purity, or 99% purity. Theterm “separating a compound” also includes the meaning of enhancing theconcentration of the compound in a mixture of such compounds, optionallycomprising solvents, such that the mixture is further enriched with adesired or preferred compound or isomer, such as an epimer, after saidseparation.

Embodiments

In a currently most preferred embodiment of the present invention Xrepresents OR₂.

In a currently preferred embodiment of the present invention R₁ and/orR₂ represent alkylsilyl, such as tert-butyldimethylsilyl, mostpreferably R₁ and R₂ are the same.

In another embodiment of the present invention R₁ and R₂ representhydrogen.

In a currently preferred embodiment of the present invention thereducing agent is a borane reducing agent, such asN,N-diethylaniline-borane, borane-tetrahydrofuran, or boranedimethylsulfide.

In a currently preferred embodiment of the invention, the reducing stepis carried out with a chiral reducing agent or in the presence of achiral auxiliary.

In a currently preferred embodiment of the present invention the chiralauxiliary is a chiral 1,2-amino-alcohol, such as a chiralcis-1-amino-2-indanol derivative, such as(1S,2R)-(−)-cis-1-amino-2-indanol.

In a currently preferred embodiment of the present invention thereducing step is carried out at a temperature between 10-20° C., inparticular 15-20° C.

In another embodiment of the present invention the molar ratio(diastereomer ratio IVa/IVb) of a mixture of compounds of generalstructure IVa and IVb, which is enriched with IVa, is larger than 55:45,such as 56:44, such as 57:43, such as 59:41, such as 60:40, such as63:37, such as 65:35, such as 68:32, such as 70:30, such as 72:28, suchas 73:27, such as 74:26, such as 75:25, such as 76:24, such as 77:23,such as 78:22, such as 79:21, such as 80:20.

In another embodiment of the present invention the molar ratio(diastereomer ratio Va/Vb) of a mixture of compounds of generalstructure Va and Vb, which is enriched with Va, is larger than 55:45,such as 56:44, such as 57:43, such as 59:41, such as 60:40, such as63:37, such as 65:35, such as 68:32, such as 70:30, such as 72:28, suchas 73:27, such as 74:26, such as 75:25, such as 76:24, such as 77:23,such as 78:22, such as 79:21, such as 80:20.

In another embodiment of the present invention the molar ratio(diastereomer ratio VIa/VIb) of a mixture of compounds of generalstructure VIa and VIb, which is enriched with VIa, is larger than 55:45,such as 56:44, such as 57:43, such as 59:41, such as 60:40, such as63:37, such as 65:35, such as 68:32, such as 70:30, such as 72:28, suchas 73:27, such as 74:26, such as 75:25, such as 76:24, such as 77:23,such as 78:22, such as 79:21, such as 80:20.

In one embodiment of the present invention, the compound of generalstructure Va is separated, e.g. by chromatography, from the mixture ofcompounds of general structure Va and Vb which is enriched with Va,wherein X, R₁ and R₂ are as defined above in (step (c)).

In another embodiment of the present invention, the compound of generalstructure VIa is separated, by chromatography, from the mixture ofcompounds of general structure VIa and VIb which is enriched with VIa,wherein X, R₁ and R₂ are as defined above in (step (g)).

Synthetic Methods

The compounds of general structure III can for example be synthesizedvia Diels-Alder reaction by treatment of a compound of general structureVII or VIII with sulphur dioxide. The sulphur dioxide used can beliquid, gaseous or being dissolved in a suitable solvent. Suitablesolvents for the Diels-Alder reaction are all solvents, which arecompatible with the reaction conditions, such as alkanes, such as hexaneor heptane, hydrocarbons, such as xylenes, toluene, ethers, such asdiethyl ether or methyl-tert-butyl ether (MTBE), acetates, such as ethylacetate or 2-propyl acetate, halogenated solvents such asdichloromethane, or mixtures of said solvents. In a preferred embodimentthe solvent is toluene. In another preferred embodiment the solvent is amixture of a water immiscible solvent and water, such as toluene andwater. The reaction can also be carried out in neat sulphur dioxidewithout a solvent. A suitable reaction temperature of the process is−50° C. to 60° C., such as −30° C. to 50° C., such as −15° C. to 40° C.,such as −5° C. to 30° C., such as 0° C. to 35° C., such as 5° C. to 30°C. most such as 10° C. to 25° C., such as 15° C. to 20° C. In oneembodiment the sulphur dioxide is used in excess (mol/mol), such as5-100 molar excess, such as 7-30 molar excess, such as 10-15 molarexcess. Any excess of unreacted sulphur dioxide may be removed from thereaction mixture by e.g. washing with aqueous base, such as aqueoussodium hydroxide or by distilling the sulphur dioxide off, optionallytogether with a solvent, optionally under reduced pressure. Thecompounds of general structure III are usually obtained as a mixture oftheir epimers IIIa and IIIb.

The molar ratio IIIa/IIIb of the mixture of the epimers obtained in theDiels-Alder reaction will depend on the groups X, R₁ and R₂ and thereaction conditions used. The present invention includes mixtures of allpossible compositions (molar ratio IIIa/IIIb), such as 1:99, such as2:98, such as 3:97, such as 4:96, such as 5:95, such as 10:90, such as85:15, such as 80:20, such as 75:25, such as 30:70, such as 35:65, suchas 40:60, such as 45:55, such as 50:50, such as 55:45, such as 60:40,such as 65:35, such as 70:30, such as 75:25, such as 80:20, such as85:10, such as 90:10, such as 95:5, such as 96:4, such as 97:3, such as98:2, such as 99:1.

The general formula III includes mixtures of all possible compositions(molar ratio IIIa/IIIb) as above. In an embodiment of the presentinvention, compounds IIIa and IIIb are used as a mixture, as indicatedin the general formula III in the following reduction step. The mixtureof IIIa and IIIb may optionally be purified or separated, such as bychromatography or crystallisation. In another embodiment compound IIIais used in the following reduction step. In yet another embodimentcompound IIIb is used in the following reduction step.

Compounds of general structure VII can for example be synthesisedaccording to methods disclosed for example by M. J. Calverley,Tetrahedron, Vol. 43, No. 20, pp. 4609-4619, 1987 or in WO 87/00834 andreferences cited therein. For example, compound VII, wherein X is OR₂and both R₁ and R₂ are tert-butyldimethylsilyl which is described inthese references can be deprotected with aqueous hydrofluoric acid inacetonitrile to give a mixture of compounds wherein X is OR₂ and eitherR₁ or R₂ are hydrogen, or to give a compound wherein X is OR₂ and R₁ andR₂ are both hydrogen. This mixture of compounds can for example beseparated by chromatography or crystallised as generally describedherein. By reaction of said compounds of general structure VII, whereinR₁ and/or R₂ are hydrogen with a suitable protecting agent, new groupsR₁ and/or R₂ can be introduced. Depending on the stoichiometry of theprotecting agent used and the reaction conditions, mixtures ofunprotected, monoprotected, and diprotected compounds can be obtained.Any intermediate of a mixture wherein X is OR₂ and one of R₁ or R₂ ishydrogen can then be isolated by chromatography and reacted withsuitable protecting agent different from the first one used, to givecompounds of general structure VII, wherein X is OR₂ and R₁ is differentfrom R₂. Compounds of general structure VII wherein X is hydrogen and R₁is hydrogen or a hydroxy protecting group can for example be preparedstarting from compound 7a and/or 7b described by M. J. Calverley,Tetrahedron, Vol. 43, No. 20, p. 4610, 1987 and following analoguesprocedures and general synthetic methods as above and as described inthe above cited references.

The reducing process of the present invention can for example be carriedout by reacting a prochiral ketone of general structure III with achiral borane reducing agent or a borane reducing agent in the presenceof a chiral auxiliary. The process results in theenantioselective/diastereoselective reduction of the prochiral ketone,such that one of the two possible epimers IVa or IVb is formed inpreference to the corresponding epimer. The degree ofenantioselectivity/diastereoselectivity will depend on the reducingagent used, the chiral auxiliary and the reaction conditions.

The reduction reaction of a compound of general structure III is usuallycarried out in a temperature interval between −80° C. to 70° C., such as−40° C. to 60° C., such as −15° C. to 50° C., such as −5° C. to 40° C.,e.g. 0° C. to 5° C. or 5° C. to 35° C. In one embodiment the temperatureinterval is between 10° C. to 30° C., such as 15° C. to 25° C., such as15° C. to 20° C. The optimum temperature will depend on the specificreaction condition and reagents used. In one embodiment of the presentinvention, the reaction mixture is immediately cooled to 0-10° C. aftercompletion to avoid the formation of by-products. If N,N-diethylanilineis used as reducing agent, the N,N-diethylaniline formed can be easilyremoved from the reaction mixture by extraction with aqueoushydrochloric acid. One molar equivalent with respect to the base to beextracted of 1M hydrochloric acid is preferred.

The reducing agent, optionally dissolved or mixed with an inert solvent,may be added to the compound of general structure III optionallydissolved or mixed with an inert solvent, e.g. under an inertatmosphere, such as nitrogen. Alternatively the compound of generalstructure III, optionally dissolved or mixed with an inert solvent, maybe added to the reducing agent, optionally dissolved or mixed with aninert solvent (reversed order).

In one embodiment of the present invention, the reducing agent is usedin an equimolar amount or in molar excess to a compound of generalstructure III. In a specific embodiment of the present invention, themolar ratio of reducing agent/compound of general structure III is1.0-5.0. In a presently preferred embodiment, the molar ratio ofreducing agent/compound of general structure III is 1.8-3.0, such as2.3-2.9, such as 2.5-2.7.

The chiral auxiliary may react with the reducing agent prior to thereduction in situ to form a chiral reducing agent or the chiralauxiliary may for example serve as a chiral ligand in a complex with thereducing agent, i.e. for example to give a chiral reducing agent. Thepresent invention includes the use of such chiral reducing agents orchiral ligand-reducing agent complexes, which were prepared and isolatedseparately before being used for the reduction of a compound of generalstructure III.

The term “reducing agent in the presence of a chiral auxiliary” thusincludes any chiral reducing agent. For example, the chiral auxiliarymay react with a borane reducing agent prior to the reduction in situ toform a chiral borane reducing agent or the chiral auxiliary may serve asa chiral ligand in a borane complex. Examples of such chiral boranereducing agents are chiral oxaborolidines or oxazaborolidines, such aschiral oxazaborolidine reagents derived from(1R,2S)-cis-1-amino-2-indanol, (1S,2R)-cis-1-amino-2-indanol,(S)-prolinol, (R)-prolinol orB-(3-pinanyl)-9-borabicyclo[3.3.2]nonane(alpine-borane), or e.g.5,5-diphenyl-2-methyl-3,4-propano-1,3,2-oxazaborolidine,(S)-2-methyl-CBS-oxazaborolidine, (R)-2-methyl-CBS-oxazaborolidine. Thepresent invention therefore includes the use of such chiral reducingagents, such as chiral borane reducing agents, or chiral ligand-reducingagent complexes, such as chiral ligand-borane complexes, which wereprepared and isolated before being used for the reduction of a compoundof general structure III.

Another example of a chiral ligand in a complex with the reducing agentis the complex of LiAlH₄ and 2,2′-dihydroxy-1,1′binaphthyl.

The molar ratio of reducing agent/chiral auxiliary is typically in therange of 0.1-20.0, such as 0.4-10.0, such as 0.3-5.0, such as 0.5-4.5,such as 1.0-4.0, such as 1.9-3.1, such as 2.1-2.9, such as 2.3-2.7, e.g.10.8, 5.4, 2.6, 2.5, or 1.6.

The chiral auxiliary may be present in catalytic amounts, such assubstoichiometric, or equimolar or in molar excess referring to acompound of general structure III or to the reducing agent. E.g. theratio of chiral auxiliary/compound III may be 0.25-2.5, such as 0.5-2.0,such as 0.8-1.3, such as 0.9-1.2, such as 1.0-1.1.

The selection of a particular enantiomer of the chiral auxiliary willdetermine the stereoselective orientation of the hydroxy group of thecompound of general structure IV with respect to C-24. Chiralauxiliaries which predominantly yield the S-configuration at C-24 arepreferred.

Borane-catalysed reactions were for example reviewed by Deloux andSrebnik [Chem. Rev. 93, 763, 1993]. Examples of efficient catalystsbased on chiral modified borane can for example be found in [A. Hirao,J. Chem. Soc. Chem. Commun. 315, 1981; E. J. Corey, J. Am. Chem. Soc.109, 7925, 1987].

Examples of the synthesis and/or use of e.g. 1,2- and 1,3-amino alcoholsin stereoselective reduction with borane can e.g. be found in [E. Didieret al.; Tetrahedron 47, 4941-4958, 1991; C. H. Senanayake et al.,Tetrahedron Letters, 36(42), 7615-18, 1995, EP 0698028, EP 0640089, EP0305180, WO 93/23408, WO 94/26751]. The synthesis and/or use of chiralcis-1-amino-2-indanol derivatives in borane reductions can e.g. be foundin [C. H. Senanayake, Aldrichimica Acta, 31 (1), 1-15, 1998; A. K. Ghoshet. al., Synthesis, 937-961, 1998; Y. Hong et. al., Tetrahedron Letters,35(36), 6631-34, 1994; B. Di Simone, Tetrahedron Asymmetry, 6(1) 301-06,1995; Y. Hong et al., Tetrahedron Letters, 36(36), 6631-34, 1994; R.Hett et al., Org. Process Res. & Dev., 2, 96-99, 1998; or EP 0763005],and references cited therein.

The methods for producing calcipotriol as described herein may bemodified with regard to the order of the reaction steps, by omitting oneor more reaction steps, or by introducing additional purification orreaction steps at any stage of the reaction sequence. The presentinvention includes all such modifications.

The method for producing calcipotriol as described herein includesfurther all variants, where the hydroxy protecting groups R₁ and/or R₂for compounds or intermediates, where R₁ and/or R₂ are not hydrogen, areremoved at any stage of the reaction sequence. Compounds orintermediates, wherein R₁ and/or R₂ are hydrogen may be protected withprotecting agents at any stage of the reaction sequence, includingprotecting agents which yield different protecting groups than thoseremoved earlier in the reaction sequence.

Compounds and intermediates of the present invention may compriseasymmetrically substituted (chiral) carbon atoms and carbon-carbondouble bonds which may give rise to the existence of isomeric forms,e.g. enantiomers, diastereomers and geometric isomers.

Epimers are known as diastereomers that have opposite configuration (Ror S) at only one of multiple tetrahedral stereogenic centres inmolecules having multiple stereogenic centres, such as the vitamin Danalogues to which the present invention is directed.

Designation of, for example, C-24 as the epimeric centre of a pair ofenantiomers therefore implies that the configuration at the otherstereogenic centres of the pair are identical.

The present invention relates to all isomeric forms, such as epimers,either in pure form or as mixtures thereof.

The indication of a specific conformation or configuration either in theformulas or the names of compounds or intermediates of the presentinvention shall indicate that this specific conformation orconfiguration is a preferred embodiment of the invention. The indicationof a specific conformation or configuration either in the formulas orthe names of compounds or intermediates of the present invention shallinclude any other isomer than specifically indicated, either in pureform or as mixtures thereof, as another embodiment of the presentinvention.

The indication of an unspecific conformation or configuration either inthe formulas or the names of compounds or intermediates of the presentinvention shall indicate that a mixture of these specific conformationsor configurations is a preferred embodiment of the invention. Forexample, the compound of general formula III is a mixture of the epimersof general formula IIIa and IIIb.

The indication of an unspecific conformation or configuration either inthe formulas or the names or numbering of compounds or intermediates ofthe present invention shall include any specific isomer although notspecifically indicated in pure form, e.g. as another embodiment of thepresent invention.

For example, the compound of general formula IVa includes the followingtwo epimers IVaa and IVab.

The meaning of compound of general formula III thus includes epimersIIIa and IIIb.

Pure stereoisomeric forms of the compounds and the intermediates of thisinvention may be obtained by the application of procedures known in theart, such as by chromatography or crystallisation, or by stereoselectivesynthesis.

The separation, isolation, and purification methods of the presentinvention include, but are not limited to chromatography, such asadsorption chromatography (including column chromatography and simulatedmoving bed (SMB)), crystallisation, or distillation. The separation,isolation, and purification methods may be used subsequently and incombination.

Column chromatography, useful for the separation of vitamin D analoguesof the present invention is well known to those skilled in the art ofpharmaceutical chemistry. The technique employs a column packed with astationary phase, for example silica, such as pretreated silica ontowhich sample to be separated is loaded. The sample is then eluted with asuitable eluent. Elution can be isocratic or so-called solventprogrammed (gradient), wherein the composition of the eluent is variedregularly (e.g. linearly) or irregularly (e.g. stepwise over time.Pretreated silica gel, well known to a person skilled in the art ofchromatography, is a suitable stationary phase. Elution with 5% (v:v)ethyl acetate in hexane or heptane followed by neat ethyl acetate is butone example of an elution program that produces the desired separation.Other suitable eluents will be deduced by the skilled person throughroutine methods of development, e.g. by using mixtures of heptane andethylacetate of suitable polarity.

For the chromatography step, any combination of stationary phase(packing) and eluent that is capable of resolving the mixture of C-24epimers can be used. Such combinations can be readily determined by theskilled person by routine experimentation. An example of a preferredstationary phase is silica, such as treated silica.

The retro Diels-Alder reaction of the mixture of compounds of generalstructure IVa and IVb, which is enriched with IVa, in the presence of abase to give a mixture of compounds of general structure Va and Vb,which is enriched with Va, wherein X, R₁, and R₂ are as defined above,may be carried out in all solvents, which are compatible with thereaction conditions, such as alkanes, such as hexane or heptane,hydrocarbons, such as xylenes, toluene, ethers, such as diethyl ether ormethyl-tert-butyl ether (MTBE), acetates, such as ethyl acetate or2-propyl acetate, halogenated solvents such as dichloromethane, water ormixtures of said solvents.

Methods of said retro Diels Alder reaction are well known to a personskilled in the art of vitamin D synthesis (see e.g. M. J. Calverley,Tetrahedron, Vol. 43, No. 20, pp. 4609-4619, 1987 or in WO 87/00834).

Preferred solvents are toluene, tert-butyl methyl ether, water, ormixtures thereof. Suitable bases to be used in the retro Diels-Alderreaction include, but are not limited to NaHCO₃, KHCO₃, Na₂CO₃, orK₂CO₃. In one embodiment of the present invention, the base is aqueousNaHCO₃ and/or the retro Diels-Alder reaction is run above 60° C., suchas above 70° C., such as between 70° C. and 120° C., such as between 74°C. and 79° C., such as between 72° C. and 78° C.

In one embodiment of the present invention, the temperature range ofextractions and phase separations after the completion of the retroDiels-Alder reaction during reaction work-up are about 30° C.-40° C.

Compounds of general structure VIII can be obtained by isomerisation ofcompounds of general structure VII.

Methods for the isomerisation of compounds of general formula Va and/orVb to VIa and/or VIb, or VII to VIII, are well known to a person skilledin the art of vitamin D synthesis. Reaction conditions can e.g. be foundin M. J. Calverley, Tetrahedron, Vol. 43, No. 20, pp. 4609-4619, 1987 orin WO 87/00834 and references cited therein. In a preferred embodimentof the present invention, the isomerisation is a photo isomerisation,e.g. with UV-light in the presence of a triplet sensitizer, e.g.anthracene or 9-acetylanthracene.

Compounds of general formula III, IV, V, VI, or VII, wherein X=hydrogenmay be hydroxylated with a suitable hydroxylating agent, for example bya selenite mediated allylic hydroxylation, under the conditionsdeveloped by Hesse, e.g. with SeO₂ and N-methylmorpholine N-oxide inrefluxing methanol and/or dichloromethane) [D. R. Andrews et al., J.Org. Chem., 1986, 51, 1637) or as described in M. J. Calverley,Tetrahedron, Vol. 43, No. 20, pp. 4609-4619, 1987 or in WO 87/00834, togive compounds of general formula III, IV, V, VI, or VII, whereinX=hydroxy (X═OR₂ and R₂=hydrogen). The hydroxy groups of the startingmaterials may be protected with suitable protecting groups such asdefined above by methods such as described above, for example to avoidundesired oxidation of said hydroxy groups.

Calcipotriol hydrate may be obtained by crystallisation of calcipotriolfrom aqueous solvents, such as for example by methods described in WO94/15912.

EXAMPLES

General:

All chemicals, unless otherwise noted were from commercial sources. Allmelting points are uncorrected. For ¹H nuclear magnetic resonance (NMR)spectra (300 MHz) and ¹³C NMR (75.6 MHz) chemical shift values (δ) (inppm) are quoted, unless otherwise specified; for deuteriochloroformsolutions relative to internal tetramethylsilane (δ=0.00) or chloroform(δ=7.26) or deuteriochloroform (δ=76.81 for ¹³C NMR) standard. The valueof a multiplet, either defined (doublet (d), triplet (t), quartet (q))or not (m) at the approximate mid point is given unless a range isquoted. All organic solvents used were of technical grade.Chromatography was performed on silica gel optionally using the flashtechnique. The TLC plates coated with silica gel were from Merck KGaA.Preferably the silica used for chromatography was from Merck KGaAGermany: LiChroprep® Si60 (15-25 μm). Ethyl acetate, dichloromethane,hexane, n-hexane, heptane or appropriate mixtures of ethyl acetate,dichloromethane, methanol, and petroleum ether (40-60), hexane orheptane were used as eluents unless otherwise noted. All reactions mayconveniently be carried out under an inert atmosphere, such as under anitrogen atmosphere.

Compounds of General Structure III Example 1 III: X═OR₂, R₁,R₂=tert-butyldimethylsilyl

1(S),3(R)-bis(tert-butyldimethylsilyloxy)-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trieneSO₂-adducts

20(R),1(S),3(R)-bis(tert-butyldimethylsilyloxy)-20-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(prepared according to the method described by M. J. Calverley,Tetrahedron, Vol. 43, No. 20, pp. 4609-4619, 1987) (20.0 g) wasdissolved in toluene (210 ml) at 20° C. followed by the addition ofwater (40 ml) and SO₂ (20 ml) with stirring. When the reaction wasjudged to be complete by HPLC {Column LiChrosorb Si 60 5 μm 250×4 mmfrom Merck, 2 ml/min flow, detection at 270 nm & mass detection,hexane/ethyl acetate 9:1 (v:v)}, usually after 2-2.5 hours, a mixture ofsodium hydroxide (27.7%, 60 ml) and water (80 ml) was added at 10-18° C.until pH 6 of the reaction mixture. The toluene phase was separated andthe solvent removed in vacuo without heating (preferably below 30° C.)to give the two epimeric SO₂-adducts IIIa and IIIb as a solid mixturepredominantly containing IIIa as checked by TLC. The two epimericSO₂-adducts IIIa and IIIb could be separated by chromatography.Crystalline IIIa could be furthermore obtained by tituration of thesolid mixture with methanol. ¹H NMR (CDCl₃) IIIa/X═OR₂, R₁,R₂=tert-butyldimethylsilyl=6.73 (dd, 1H), 6.14 (d, 1H), 4.69 (d, 1H),4.62 (d, 1H), 4.35 (s, 1H), 4.17 (m, 1H), 3.92 (d, 1H), 3.58 (d, 1H),2.61 (m, 1H), 2.29 (m, 1H), 2.2-1.2 (m, 16H), 1.11 (d, 3H), 1.05 (m,2H), 0.90 (m, 2H), 0.87 (s, 9H), 0.85 (s, 9H), 0.68 (s, 3H), 0.06 (s,3H), 0.05 (s, 3H), 0.04 (s, 3H), 0.02 (s, 3H) ppm.

Preparation 1:

VII: X═OR₂, R₁, R₂=hydrogen

1(S),3(R)-dihydroxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

20(R),1(S),3(R)-bis(tert-butyldimethylsilyloxy)-20-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trienewhich was obtained according to the method described by M. J. Calverley,Tetrahedron, Vol. 43, No. 20, p. 4614-4619, 1987 was dissolved inacetonitrile. Aqueous hydrofluoric acid (40%) was added and the mixturewas stirred at room temperature for ca. 1 hour. The progress of thereaction could be conveniently checked by TLC using ethyl acetate as aneluent. Ethyl acetate was added to the reaction mixture and the mixturewas washed with aqueous sodium hydrocarbonate solution. The organicphase was dried with over MgSO₄ and concentrated. The crystals (whiteneedles) which formed were filtered off, washed with ethyl acetate, anddried in vacuo to give the title compound VII (X═OR₂, R₁, R₂=hydrogen).¹H NMR (CDCl₃) VII/X═OR₂, R₁, R₂=hydrogen=6.77 (dd, 1H), 6.57 (d, 1H),6.15 (d, 1H), 5.88 (dd, 1H), 5.13 (dd, 1H), 4.98 (s, 1H), 4.50 (m, 1H),4.23 (m, 1H), 2.86 (m, 2H), 2.29 (m, 2H), 2.14-1.20 (m, 16H), 1.14 (d,3H), 1.08 (m, 2H), 0.89 (m, 2H), 0.61 (s, 3H), ppm.

Example 2 III: X═OR₂, R₁, R₂=hydrogen

1(S),3(R)-dihydroxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trieneSO₂-adducts.

Same method as in Example 1, except that the starting material was1(S),3(R)-dihydroxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trienefrom preparation 1. ¹H NMR (CDCl₃) III/X═OR₂, R₁, R₂=hydrogen δ=6.80(dd, 1H), 6.15 (d, 1H), 4.75 (m, 2H), 4.5-3.9 (m, 4H), 3.70 (d, 1H),2.60 (m, 1H), 2.5-0.8 (m, 25H), 0.68 (s, 3H), ppm; ¹³C NMR (CDCl₃)III/X═OR₂, R1, R2=hydrogen δ=201.0, 152.1, 151.0, 133.7, 129.2, 128.3,108.8, 67.3, 65.1, 63.6, 56.1, 55.9, 55.5, 46.5, 40.1, 39.9, 33.9, 29.8,27.4, 23.9, 22.1, 19.5, 18.9, 12.2, 11.2 ppm.

Preparation 2:

VII: X═OR₂, R₁=hydrogen, R₂=tert-butyldimethylsilyl

1(S)-tert-butyldimethylsilyl-3(R)-hydroxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene.

20(R),1(S),3(R)-bis(tert-butyldimethylsilyloxy)-20-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trienewas partially deprotected using the same deprotection conditions as usedin Preparation 1 giving a mixture of unreacted starting material, twopartially deprotected intermediates and the compound of Preparation 1.Purification by chromatography gave the pure title compound.

The ¹H NMR was found to be in accordance with the structure. ¹H NMR(CDCl₃) VII/X═OR₂, R₁=hydrogen, R₂=tert-butyldimethylsilyl δ=6.75 (dd,1H), 6.50 (d, 1H), 6.14 (d, 1H), 5.84 (d, 1H), 5.00 (s, 1H), 4.92 (s,1H), 4.47 (t, 1H), 4.22 (m, 1H), 2.85 (dd, 1H), 2.62 (dd, 1H), 2.43 (dd,1H), 2.29 (m, 1H), 2.15-1.15 (m, 15H), 1.11 (d, 3H), 1.06 (m, 2H), 0.87(s, 9H), 0.86 (m, 2H), 0.59 (s, 3H), 0.06 (s, 3H), 0.04 (s, 3H), ppm.

Example 3 III: X═OR₂, R₁=hydrogen, R₂=tert-butyldimethylsilyl

1(S)-tert-butyldimethylsilyl-3(R)-hydroxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trieneSO₂-adducts.

Same method as in Example 1, except that the starting material was1(S)-tert-butyldimethylsilyl-3(R)-hydroxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trienefrom preparation 2. ¹³C NMR (CDCl₃) III/X═OR₂, R₁=hydrogen,R₂=tert-butyldimethylsilyl δ=200.3, 151.5, 150.4, 132.0, 129.5, 128.0,108.5, 66.8, 65.5, 63.8, 56.1, 55.9, 55.2, 46.2, 39.8, 33.6, 29.5, 27.2,25.4, 23.7, 21.8, 19.2, 18.5, 17.7, 11.8, 10.7, −4.7, −5.2 ppm; ¹H NMR(CDCl₃) IIIb/X═OR₂, R₁=hydrogen, R₂=tert-butyldimethylsilyl δ=6.75 (dd,1H), 6.14 (d, 1H), 4.80 (d, 1H), 4.65 (d, 1H), 4.43 (m, 1H), 4.25 (m,1H), 3.92 (d, 1H), 3.63 (dd, 1H), 2.60 (d, 1H), 2.5-1.2 (m, 18H), 1.12(d, 3H), 1.06 (m, 2H), 0.88 (s, 9H), 0.87 (m, 2H), 0.59 (s, 3H), 0.09(s, 3H), 0.07 (s, 3H), ppm.

Preparation 3:

VII: X═OR₂, R₁═COCMe₃, R₂=tert-butyldimethylsilyl

1(S)-tert-butyldimethylsilyl-3(R)-trimethylacetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

1(S)-tert-butyldimethylsilyl-3(R)-hydroxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trienefrom Preparation 2 may be reacted with trimethylacetic acid chloride inthe presence of triethylamine in dichloromethane. The obtained rawproduct may be purified by chromatography to give the pure titlecompound.

Preparation 4:

VII: X═OR₂, R₁═COCMe₃, R₂=hydrogen

1(S)-hydroxy-3(R)-trimethylacetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene.

1(S)-tert-butyldimethylsilyl-3(R)-trimethylacetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trienemay be deprotected using the same deprotection conditions as used inPreparation 1. The obtained raw product may be purified bychromatography to give the pure title compound.

Example 4 III: X═OR₂, R₁═COCMe₃, R₂=hydrogen

1(S)-hydroxy-3(R)-trimethylacetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trieneSO₂-adducts.

Same method as in example 1, except that the starting material was1(S)-hydroxy-3(R)-trimethylacetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trienefrom preparation 4. ¹³C NMR (CDCl₃) IIIa/X═OR₂, R₁═COCMe₃, R₂=hydrogenδ=200.4, 177.6, 151.6, 150.9, 132.8, 129.3, 128.1, 108.8, 66.9, 66.3,64.6, 55.8, 55.5, 55.3, 46.3, 39.9, 38.5, 36.3, 30.2, 29.6, 27.2, 26.9,23.7, 21.8, 19.3, 18.6, 11.9, 10.8 ppm.

Example 5 III: X═OR₂, R₁═COCMe₃, R₂=tert-butyldimethylsilyl

-   1(S)-tert-butyldimethylsilyl-3(R)-trimethylacetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene    SO₂-adducts.

Same method as in Example 1, except that the starting material was1(S)-tert-butyldimethylsilyl-3(R)-trimethylacetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trienee.g. obtainable from Preparation 3.

Preparation 5:

VII: X═OR₂, R₁═COMe, R₂=hydrogen

1(S)-hydroxy-3(R)-acetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene.

1(S),3(R)-dihydroxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(VII: X═OR₂, R₁, R₂=hydrogen) from Preparation 1 may be reacted with oneequivalent acetylchloride in the presence of triethylamine. The mixtureof products may be purified by chromatography on silica to give the puretitle compound.

Example 6 III: X═OR₂, R₁═COMe, R₂=hydrogen

1(S)-hydroxy-3(R)-acetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-trieneSO₂-adducts.

Same method as in Example 1, except that the starting material was1(S)-hydroxy-3(R)-acetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(VII: X═OR₂, R₁═COMe, R₂=hydrogen) from obtainable from Preparation 5.¹³C NMR (CDCl₃) IIIa/X═OR₂, R₁═COMe, R₂=hydrogen δ=200.5, 170.3, 151.6,150.9, 132.8, 129.2, 128.1, 108.3, 66.8, 66.4, 64.6, 55.9, 55.7, 55.3,46.3, 39.9, 36.4, 30.4, 29.6, 27.2, 23.7, 21.8, 21.0, 19.3, 18.6, 11.9,10.8.

Example 7 III: X═OR₂, R₁═COMe, R₂=tert-butyldimethylsilyl

-   1(S)-tert-butyldimethylsilyl-3(R)-acetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene    SO₂-adducts.

Same method as in Example 1, except that the starting material was1(S)-tert-butyldimethylsilyl-3(R)-acetoxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene.¹H NMR (CDCl₃) IIIa/X═OR₂, R₁═COMe, R₂=tert-butyldimethylsilyl δ=6.75(dd, 1H), 6.16 (d, 1H), 5.20 (m, 1H), 4.71 (s, 2H), 4.33 (s, 1H), 3.95(d, 1H), 3.60 (d, 1H), 2.61 (m, 1H), 2.31 (m, 2H), 2.15-1.2 (m, 15H),2.03 (s, 3H), 1.11 (d, 3H), 1.07 (m, 2H), 0.89 (m, 2H), 0.88 (s, 9H),0.68 (s, 3H), 0.08 (s, 3H), 0.07 (s, 3H), ppm.

Example 8 III: X=hydrogen, R₁=tert-butyldimethylsilyl

-   3(R)-tert-butyldimethylsilyloxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene    SO₂-adducts

The starting material VII, X=hydrogen, R₁=tert-butyldimethylsilyl(prepared according to the methods described by M. J. Calverley,Tetrahedron, Vol. 43, No. 20, pp. 4609-4619, 1987) (38.5 g) wasdissolved in toluene (550 ml) at 20° C. followed by the addition ofwater (105 ml) and SO₂ (53 ml) with stirring. When the reaction wasjudged to be complete by HPLC {Column LiChrosorb Si 60 5 μm 250×4 mmfrom Merck, 2 ml/min flow, detection at 270 nm & mass detection,hexane/ethyl acetate 9:1 (v:v)}, usually after 2-2.5 hours, a mixture ofsodium hydroxide (27.7%, 150 ml) and water (480 ml) was added at 10-18°C. until pH 6 of the reaction mixture. The toluene phase was separatedand the solvent removed in vacuo without heating (preferably below 30°C.) to give two epimeric SO₂-adducts IIIa and IIIb (X=hydrogen,R₁=tert-butyldimethylsilyl) as a solid mixture predominantly containingIIIa as checked by TLC. The two epimeric SO₂-adducts could be separatedby chromatography. Crystalline IIIa could be furthermore obtained bytituration of the solid mixture with methanol. ¹³C NMR (CDCl₃) (III:X=hydrogen, R₁=tert-butyldimethylsilyl, mixture of isomers IIIa andIIIb): 200.3, 151.6, 151.4, 149.8, 149.2, 130.5, 130.1, 128.3, 128.1,126.6, 126.3, 110.5, 110.0, 67.4, 66.7, 66.6, 66.3, 58.0, 57.9, 55.8,55.6, 55.3, 55.2, 46.3, 45.5, 39.9, 39.7, 34.4, 34.1, 33.9, 31.4, 30.8,30.5, 29.6, 29.1, 27.3, 27.1, 26.7, 25.6, 25.1, 24.4, 24.1, 23.6, 23.2,22.4, 21.9, 21.9, 19.4, 19.3, 18.6, 18.4, 17.9, 17.9, 13.9, 12.2, 11.9,10.8, −5.0.

Compounds of General Structure IV Example 9 SO₂-adduct of1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3′(S)-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

(IVa: X═OR₂, R₁, R₂=tert-butyldimethylsilyl), and

SO₂-adduct of1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(R)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

(IVb: X═OR₂, R₁, R₂=tert-butyldimethylsilyl)

(1S,2R)-(−)-cis-1-amino-2-indanol (5.0 g) was mixed with MTBE (160 ml)under a nitrogen atmosphere at 15-25° C. followed by the addition ofN,N-diethylaniline-borane (16.0 ml) at that temperature. The mixture wasstirred until no more evolution of hydrogen could be observed. Themixture of SO₂-adducts of1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(III: X═OR₂, R₁, R₂=tert-butyldimethylsilyl) obtained in Example 1 wasdissolved in toluene (160 ml) and MTBE (80 ml). This solution was addeddropwise to the borane containing mixture at 15-25° C. The mixture wasstirred for ca. 30-60 minutes after complete addition and then quenchedwith saturated aqueous NaHCO₃ (110 ml) at 10-15° C. The organic phasewas separated and washed with 1 M hydrochloric acid (100 ml) at 0-10° C.followed by washing with saturated aqueous NaHCO₃ (100 ml). The organicphase contained the SO₂-adducts of1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(IVa: X═OR₂, R1, R2=tert-butyldimethylsilyl), and the SO₂-adducts of1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(R)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(IVb: X═OR₂, R₁, R₂=tert-butyldimethylsilyl) in a molar ratio of72-78:22-28 (IVa:IVb) as checked by HPLC-analysis of an aliquot afterretro-Diels Alder reaction and analysis according to the methoddescribed in example 10}. Compound IVaa was isolated by chromatographyon silica. ¹³C NMR (CDCl₃) IVa/X═OR₂, R₁, R₂=tert-butyldimethylsilylδ=150.6, 137.6, 132.3, 129.3, 128.8, 109.0, 76.9, 67.3, 65.8, 64.5,56.2, 56.1, 55.9, 46.0, 40.5, 40.0, 39.6, 34.1, 29.6, 27.4, 25.6, 25.5,23.8, 21.8, 20.3, 17.8, 17.7, 17.4, 11.8, 2.8, 1.7, −4.7, −5.0, −5.0,−5.2 ppm.

Example 10 SO₂-adducts of3(R)-tert-butyl-dimethylsilyloxy-20(R)-(3′-cyclopropyl-3′(S)-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

(IVa: X=hydrogen, R₁=tert-butyldimethylsilyl), and

-   SO₂-adducts of    3(R)-tert-butyl-dimethylsilyloxy-20(R)-(3′-cyclopropyl-3′(R)-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

(IVb: X=hydrogen, R₁=tert-butyldimethylsilyl),

(1S,2R)-(−)-cis-1-amino-2-indanol (1.22 g, 1.08 eq.) was mixed with MTBE(36 ml) under a nitrogen atmosphere at 15-25° C. followed by theaddition of N,N-diethylaniline-borane (3.6 ml, 2.7 eq.) at thattemperature. The mixture was stirred until no more evolution of hydrogencould be observed. The mixture of SO₂-adducts of3(R)-tert-butyl-dimethylsilyloxy-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(III: X=hydrogen, R₁=tert-butyldimethylsilyl) obtained in Example 8(4.32 g) was dissolved in a mixture of MTBE (18 ml) and toluene (36 ml)at room temperature and then added dropwise to the borane containingmixture at 15-25° C. over 15 min. The mixture was stirred for ca. 60minutes after complete addition and then quenched with saturated aqueousNaHCO₃ (25 ml). The organic phase was separated and washed with 1 Mhydrochloric acid (25 ml) at 0-10° C. followed by washing with saturatedaqueous NaHCO₃ (25 ml) at 10-20° C. The organic phase contained theSO₂-adducts of3(R)-tert-butyl-dimethylsilyloxy-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(IVa: X=hydrogen, R₁,=tert-butyldimethylsilyl), and the SO₂-adducts of3(R)-tert-butyl-dimethylsilyloxy-20(R)-(3′-cyclopropyl-3(R)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(IVb: X=hydrogen, R₁=tert-butyldimethylsilyl).

Compounds of General Structure V Example 111(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

(Va: X═OR₂, R₁, R₂=tert-butyldimethylsilyl), and

1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(R)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

(Vb: X═OR₂, R₁, R₂=tert-butyldimethylsilyl)

The organic phase from Example 9 containing the SO₂-adducts of IVa(X═OR₂, R₁, R₂=tert-butyldimethylsilyl), and IVb (X═OR₂, R₁,R₂=tert-butyldimethylsilyl) was stirred vigorously with saturatedaqueous NaHCO₃ (110 ml) and then heated (bath temperature ca. 90° C.)where the MTBE was distilled off. Conveniently the retro Diels-Alderreaction could be checked by HPLC HPLC {Column LiChrosorb Si 60 250×4 mmfrom Merck, 1 ml/min flow, detection at 270 nm, hexane/ethyl acetate9:1.5 (v:v)}. After completion (usually 2-2.5 hours), the reactionmixture was cooled to 30-40° C. and the organic phase was separated,washed with saturated aqueous NaHCO₃ (110 ml) and water (100 ml). Thesolvent was removed in vacuo and the obtained oil (29 g) was dissolvedin hexane (200 ml). The organic mixture was cooled to ca. −15° C.,filtered over a short path of silica, and the remainder washed withhexane (ca. 100 ml). The hexane phase was washed with a mixture ofmethanol and water (1:2) and the organic solvent was removed in vacuo.The remaining oil, containing a mixture of1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(Va: X═OR₂, R₁, R₂=tert-butyldimethylsilyl), and1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(R)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(Vb: X═OR₂, R₁, R₂=tert-butyldimethylsilyl) in a molar ratio of a rangeof 72-78:22-28 (Va:Vb) as checked by HPLC {Column LiChrosorb Si 60 5 μm250×4 mm from Merck, 1 ml/min flow, detection at 270 nm,n-heptane/2-propanol 100:0.25 (v:v): RT Va ca. 14.3 min, Vb: 11.9 min;or hexane/ethyl acetate 90:15 (v:v): RT Va ca. 7.6 min, Vb: 6.4 min},was purified by chromatography as described earlier by M. J. Calverley,Tetrahedron, Vol. 43, No. 20, pp. 4609-4619, 1987 or in WO 87/00834, togive 10.9 g (98.9% HPLC purity) of Va/X═OR₂, R₁,R₂=tert-butyldimethylsilyl after crystallisation from a mixture ofhexane and methanol and a small amount of triethylamine (by slowlyevaporating the hexane followed by cooling to −15° C.), in fullaccordance with the data described by M. J. Calverley in Tetrahedron,Vol. 43, No. 20, p. 4617, 1987 for compound 22. ¹³C NMR (CDCl₃)Va/X═OR₂, R₁, R₂=tert-butyldimethylsilyl δ=153.4, 142.9, 137.9, 135.2,128.7, 121.5, 116.3, 106.4, 77.1, 70.0, 67.0, 56.2, 55.8, 45.7, 43.7,40.2, 39.8, 36.3, 28.7, 27.5, 25.6, 25.6, 23.3, 22.0, 20.3, 18.0, 17.9,17.4, 12.1, 2.9, 1.6, −5.0, −5.0, −5.1; Vb/X═OR₂, R₁,R₂=tert-butyldimethylsilyl, δ=153.5, 142.9, 137.6, 135.3, 128.7, 121.5,116.3, 106.4, 76.8, 70.0, 67.0, 56.2, 56.0, 45.7, 43.8, 40.2, 39.7,36.4, 28.7, 27.6, 25.7, 25.6, 23.3, 22.0, 20.3, 18.0, 17.9, 17.3, 12.1,2.8, 1.6, −5.0, −5.1, −5.1 ppm.

Example 123(R)-tert-butyl-dimethylsilyloxy-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

(Va: X=hydrogen, R₁=tert-butyldimethylsilyl), and

3(R)-tert-butyl-dimethylsilyloxy-20(R)-(3′-cyclopropyl-3(R)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene

(Vb: X=hydrogen, R₁=tert-butyldimethylsilyl)

The organic solution from Example 10 containing the SO₂-adducts of IVa(X=hydrogen, R₁=tert-butyldimethylsilyl), and IVb (X=hydrogen,R₁=tert-butyldimethylsilyl) was stirred vigorously with saturatedaqueous NaHCO₃ (25 ml) and then heated (bath temperature ca. 90° C.)where the MTBE was distilled off. Conveniently the retro Diels-Alderreaction could be checked by HPLC HPLC {Column LiChrosorb Si 60 250×4 mmfrom Merck, 1 ml/min flow, detection at 270 nm, hexane/ethyl acetate9:1.5 (v:v)}. After completion (approx. 2 hours), the reaction mixturewas cooled to 15-25° C. and the organic phase was separated, washed withwater (25 ml)., containing a mixture of Va:Vb (X=hydrogen,R₁=tert-butyldimethylsilyl) in a molar ratio of (75:25) as checked byHPLC {Column LiChrosorb Si 60 5 μm 250×4 mm from Merck, 1 ml/min flow,detection at 270 nm, hexane/ethylacetate 90:15 (v:v): RT Vb: ca. 6.1min, RT Va: ca. 7.4 min}. ¹H NMR (CDCl₃) Va/X=hydrogen,R₁=tert-butyldimethylsilyl δ=6.45 (d, 1H), 5.84 (d, 1H), 5.46 (m, 2H),4.92 (s, 1H), 4.63 (s, 1H), 3.84 (m, 1H), 3.42 (m, 1H), 2.85 (d, 1H),2.64 (d, 1H), 2.45 (m, 1H), 2.32-1.18 (m, 17H), 1.04 (d, 3H), 0.98 (m,1H), 0.87 (s, 9H), 0.56 (s, 3H), 0.51 (m, 2H), 0.32 (m, 1H), 0.22 (m,1H), 0.05 (s, 3H), 0.04 (s, 3H); Vb/X=hydrogen,R₁=tert-butyldimethylsilyl δ=6.45 (d, 1H), 5.83 (d, 1H), 5.47 (m, 2H),4.90 (s, 1H), 4.62 (s, 1H), 3.83 (m, 1H), 3.45 (m, 1H), 2.83 (d, 1H),2.62 (d, 1H), 2.44 (m, 1H), 2.24 (m, 1H), 2.18-1.17 (m, 16H), 1.03 (d,3H), 0.96 (m, 1H), 0.86 (s, 9H), 0.55 (s, 3H), 0.50 (m, 2H), 0.30 (m,1H), 0.20 (m, 1H), 0.05 (s, 3H), 0.04 (s, 3H).

Compounds of General Structure VI Example 131(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(Z),7(E),10(19)-triene

(X═OR₂, VIa: R₁, R₂=tert-butyldimethylsilyl)

1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene(Va: X═OR₂, R₁, R₂=tert-butyldimethylsilyl) obtained in Example 11 wasphotosisomerised in toluene using a high pressure ultraviolet lamp at20° C. as described earlier by M. J. Calverley, Tetrahedron, Vol. 43,No. 20, pp. 4609-4619, 1987 or in WO 87/00834, except that9-acetylanthracene was used instead of anthracene, to give1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(Z),7(E),10(19)-triene(VIa: X═OR₂, R₁, R₂=tert-butyldimethylsilyl) after chromatography infull accordance with the data described by M. J. Calverley inTetrahedron, Vol. 43, No. 20, p. 4618, 1987 for compound 28.

Calcipotriol Example 141(S),3(R)-dihydroxy-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(Z),7(E),10(19)-triene

1(S),3(R)-bis(tert-butyl-dimethylsilyloxy)-20(R)-(3′-cyclopropyl-3(S)′-hydroxyprop-1′(E)-enyl)-9,10-secopregna-5(Z),7(E),10(19)-triene(VIa: X═OR₂, R₁, R₂=tert-butyldimethylsilyl) obtained in Example 13 wasdeprotected using tetrabutyl ammonium fluoride in tetrahydrofuran at 60°C. followed by chromatography, as described earlier by M. J. Calverley,Tetrahedron, Vol. 43, No. 20, pp. 4609-4619, 1987 or in WO 87/00834.Crystallisation from ethylacetate/hexane containing a few drops oftriethylamine gave calcipotriol in full accordance with the datadescribed by M. J. Calverley in Tetrahedron, Vol. 43, No. 20, p. 4618,1987 for compound 4.

Calcipotriol Monohydrate Example 15

The calcipotriol obtained in Example 14 was crystallised from ethylacetate/water as described in WO 94/15912 to give calcipotriolmonohydrate in full accordance with the characteristic data described inthat patent.

Diastereoselective Reduction Under Various Reducing Conditions Example16

TABLE 1 Diastereoselective reduction of compounds of general structureIII, where X = OR₂ and R₁ and R₂ = tert-butyldimethylsilyl (mixture of1(S),3(R)-bis(tert-butyldimethylsilyloxy)-20(R)-(3′-cyclopropyl-3′-oxoprop-1′(E)-enyl)-9,10-secopregna-5(E),7(E),10(19)-triene SO₂-adductsfrom Example 1 following a procedure analogous to Example 9 followed bycheletropic extrusion of sulfur dioxide following a procedure analogousto Example 11 to yield compounds of general structure. Va: X = OR₂, R₁,R₂ = tert-butyldimethylsilyl and Vb: X = OR₂, R₁, R₂ = tert-butyldimethylsilyl under various conditions (eq. = molar equivalentsrelative to III; MTBE = tert-butylmethyl ether; DEANB = N,N-diethylaniline borane). Reducing Ratio Chiral Auxiliary reagent Temp.Va:Vb (eq.) (eq.) (° C.) Solvent (%) (1S,2R)-(−)-cis-1-amino- DEANB15-20 MTBE/ 72:28 2-indanol (1.1 eq.) (2.7 eq.) toluene(1S,2R)-(−)-cis-1-amino- DEANB 20-25 MTBE/ 72:28 2-indanol (1.1 eq.)(2.7 eq.) toluene (1S,2R)-(−)-cis-1-amino- DEANB 10-15 MTBE/ 70:302-indanol (1.1 eq.) (2.7 eq.) toluene (1S,2R)-(−)-cis-1-amino- DEANB15-20 MTBE/ 72:28 2-indanol (0.5 eq.) (2.7 eq.) toluene(1S,2R)-(−)-cis-1-amino- DEANB 15-20 MTBE/ 56:44 2-indanol (0.25 eq.)(2.7 eq.) toluene (1S,2R)-(−)-cis-1-amino- DEANB 15-20 MTBE/ 59:412-indanol (1.1 eq.) (1.8 eq.) toluene (1S,2R)-(−)-cis-1-amino- BH₃•THF15-20 MTBE/ 75:25 2-indanol (1.1 eq.) (2.7 eq.) toluene(1S,2R)-(−)-cis-1-amino- BH₃•SMe₂ 15-20 MTBE/ 73:27 2-indanol (1.1 eq.)(2.7 eq.) toluene (1S,2R)-(−)-cis-1-amino- DEANB 15-20 THF 63:372-indanol (1.1 eq.) (2.7 eq.) (R)-(+)-α,α-Diphenyl-2- DEANB 15-20 MTBE/68:32 pyrrolidinmethanol (1 eq.) (2.7 eq.) toluene (R)-(+)-2-Amino-4-DEANB 15-20 MTBE/ 72:28 methyl-1,1-diphenyl-1- (2.7 eq.) toluenepentanol (0.5 eq.) (R)-(−)-2-Amino-3- DEANB 15-20 MTBE/ 76:24methyl-1,1-diphenyl-1- (2.7 eq.) toluene butanol (0.5 eq.)(R)-(+)-2-amino-1,1,3- DEANB 15-20 MTBE/ 74:26 triphenyl-1-propanol (0.5eq.) (2.7 eq.) toluene (1R,2S)-(−)-2-Amino-1,2- DEANB 15-20 MTBE/ 57:43diphenyl ethanol (0.5 eq.) (2.7 eq.) toluene

1. A method of reducing a compound of structure III,

wherein X represents either hydrogen or OR₂, and wherein R₁ and R₂ maybe the same or different and represent hydrogen, or a hydroxy protectinggroup, in an inert solvent with a chiral reducing agent or with areducing agent in the presence of a chiral auxiliary, to give a mixtureof compounds of structure IVa and IVb,

which is enriched with IVa, wherein X, R₁, and R₂ are as defined above.2. A method for producing calcipotriol{(5Z,7E,22E,24S)-24-cyclopropyl-9,10-secochola-5,7,10(19),22-tetraene-1α-3β-24-triol}or calcipotriol monohydrate comprising the steps of: (a) reducing acompound of structure III according to claim 1,

wherein X represents OR₂, and wherein R₁ and R₂ may be the same ordifferent and represent hydrogen or a hydroxy protecting group, in aninert solvent with a chiral reducing agent or with a reducing agent inthe presence of a chiral auxiliary, to give a mixture of compounds ofstructure IVa and IVb, which is enriched with IVa,

wherein X, R₁ and R₂ are as defined above; (b) reacting the mixture ofcompounds of structure IVa and IVb, which is enriched with IVa, in thepresence of a base to give a mixture of compounds of structure Va andVb, which is enriched with Va,

wherein X, R₁ and R₂ are as defined above; (c) separating the compoundof structure Va from the mixture of compounds of structure Va and Vbwhich is enriched with Va, wherein X, R₁ and R₂ are as defined above;(d) isomerising the compound of structure Va to the compound ofstructure VIa,

wherein X, R₁ and R₂ are as defined above; and (e) when R₁ and/or R₂ arenot hydrogen, removing the hydroxy protecting group(s) R₁ and/or R₂ ofthe compound of structure VIa to generate calcipotriol or calcipotriolmonohydrate.
 3. A method for producing calcipotriol or calcipotriolmonohydrate comprising steps (a)-(b) of claim 2 and further comprisingthe steps of: (f) isomerising the mixture of compounds of structure Vaand Vb, wherein X, R₁ and R₂ are as defined in claim 2, which isenriched with Va, to a mixture of compounds of structure VIa and VIb,which is enriched with VIa,

wherein X, R₁ and R₂ are as defined above; (g) separating the compoundof structure VIa from the mixture of compounds of structure VIa and VIbwhich is enriched with VIa, wherein X, R₁ and R₂ are as defined above;(h) when R₁ and/or R₂ are not hydrogen, removing the hydroxy protectinggroup(s) R₁ and/or R₂ of the compound of structure VIa to generatecalcipotriol or calcipotriol monohydrate.
 4. A method for producingcalcipotriol{(5Z,7E,22E,24S)-24-cyclopropyl-9,10-secochola-5,7,10(19),22-tetraene-1α-3β-24-triol}or calcipotriol monohydrate comprising the steps of: (j) reducing acompound of structure III according to claim 1,

wherein X represents hydrogen, and wherein R₁ represents hydrogen or ahydroxy protecting group, in an inert solvent with a chiral reducingagent or with a reducing agent in the presence of a chiral auxiliary, togive a mixture of compounds of structure IVa and IVb, which is enrichedwith IVa,

wherein X and R₁ are as defined above; (k) reacting the mixture ofcompounds of structure IVa and IVb, which is enriched with IVa, in thepresence of a base to give a mixture of compounds of structure Va andVb, which is enriched with Va,

wherein X and R₁ are as defined above; (l) separating the compound ofstructure Va from the mixture of compounds of structure Va and Vb whichis enriched with Va, wherein X and R₁ are as defined above; (m)hydroxylating the compound of structure Va with a suitable hydroxylatingagent, wherein X and R₁ are as defined above to give a compound ofstructure Va, wherein X represents OR₂ and R₂ represents hydrogen, andwherein R₁ is as defined above; (o) isomerising the compound ofstructure Va to the compound of structure VIa,

wherein X, R₁ and R₂ are as defined above; and (p) when R₁ is nothydrogen, removing the hydroxy protecting group R₁ of the compound ofstructure VIa to generate calcipotriol or calcipotriol monohydrate.
 5. Amethod for producing calcipotriol or calcipotriol monohydrate comprisingsteps (j)-(l) of claim 4 and further comprising the steps of: (q)protecting the C-24 hydroxy group of the compound of structure Va,

wherein X represents hydrogen, and wherein R₁ represents hydrogen or ahydroxy protecting group, with a hydroxy protecting group; (r)hydroxylating the C-24 hydroxy protected compound of structure Va with asuitable hydroxylating agent, wherein X and R₁ are as defined above togive a C-24 hydroxy protected compound of structure Va, wherein Xrepresents OR₂ and R₂ represents hydrogen, and wherein R₁ is as definedabove; (s) removing the C-24 hydroxy protecting group of the compound ofstructure Va; (t) isomerising the compound of structure Va to thecompound of structure VIa,

wherein X, R₁ and R₂ are as defined above; and (u) when R₁ is nothydrogen, removing the hydroxy protecting group R₁ of the compound ofstructure VIa to generate calcipotriol or calcipotriol monohydrate. 6.The method according to claim 1, wherein the reducing agent is a boranederivative.
 7. The method according to claim 1, wherein the reducingstep is with a reducing agent in the presence of a chiral auxiliary andwherein the reducing agent is N,N-diethylaniline-borane,borane-tetrahydrofuran, or borane dimethylsulfide.
 8. The methodaccording to claim 1, wherein the reducing step is with a reducing agentin the presence of a chiral auxiliary and wherein the chiral auxiliaryis a chiral 1,2-amino-alcohol.
 9. The method according to claim 1,wherein the reducing step is with a reducing agent in the presence of achiral auxiliary and wherein the chiral auxiliary is a chiralcis-1-amino-2-indanol derivative.
 10. The method according to claim 1,wherein the reducing step is with a reducing agent in the presence of achiral auxiliary and wherein the chiral auxiliary is(1S,2R)-(−)-cis-1-amino-2-indanol.
 11. The method according to claim 1,wherein the inert solvent is toluene, tert-butyl methyl ether,tetrahydrofuran, or mixtures thereof.
 12. The method according to claim1, wherein the mixture of compounds of structure IVa and IVb obtained byreducing a compound of structure III has a molar ratio of IVa:IVb whichis at least 56:44.
 13. The method according to claim 10, wherein thereducing step is carried out at a temperature between 10-20° C.
 14. Amethod of reacting the mixture of compounds of structure IVa and IVb,

wherein X represents either hydrogen or OR₂, and wherein R₁ and R₂ maybe the same or different and represent hydrogen, or a hydroxy protectinggroup, which is enriched with IVa, in the presence of a base to give amixture of compounds of structure Va and Vb, which is enriched with Va,

wherein X, R₁, and R₂ are as defined above.
 15. A method according toclaim 1, wherein X represents OR₂.
 16. A method according to claim 15,wherein R₁ and/or R₂ represent alkylsilyl.
 17. A method according toclaim 15, wherein R₁ and/or R₂ represent tert-butyldimethylsilyl.